1. Field of the Invention
The present invention relates to a suspension system of a motor vehicle for supporting a vehicle body on each axle.
2. Description of the Related Art
A suspension system comprises suspensions which are arranged individually between the body and axles of a motor vehicle. Generally known, among suspensions of various types, is an air suspension in which the damping force of shock absorber or the spring constant of air spring can be controlled electronically.
Examples of systems which are provided with the air suspensions of this type are described in Published Unexamined Japanese Patent Applications Nos. 3-208712 and 3-208713. According to the former system, the roll of the vehicle body can be reduced by varying the spring constant of the each air spring. According to the latter system, on the other hand, the roll of the vehicle body can be reduced by varying the damping force of the each air spring or shock absorber.
In reducing the roll of the vehicle body by means of the air suspensions, according to both these systems, air suspensions on the rear-wheel side are first controlled so as to reduce the roll of the rear portion of the vehicle body, and then, after the passage of a predetermined time, air suspensions on the front-wheel side are controlled so as to reduce the roll of the front portion of the vehicle body.
When the roll control for the front portion of the vehicle body is started after the passage of the predetermined time from the start of the roll control for the rear portion of the vehicle body, the roll stiffness of the rear portion first increases immediately after the start of turn of the motor vehicle. Further, the steering characteristic of the vehicle in the initial stage of the turn shifts to the over-steering side, so that the vehicle is allowed to turn with ease.
When the roll control for the front portion of the vehicle body is started after the passage of the predetermined time, the roll stiffness of the front portion also starts to increase, and the steering characteristic of the motor vehicle shifts to the under-steering side, in contrast with the case of the initial stage of the turn. Thus, the motor vehicle steadily turns in the middle and final stages.
The roll control for the vehicle body by means of the suspension system described above is executed on the assumption that the motor vehicle is running on a level road. Accordingly, the delay time or time interval between the start of the roll control for the rear portion of the vehicle body and the start of the roll control for the front portion can be varied only on the basis of the vehicle velocity.
However, the motor vehicle runs not only on a level road but also on an upward or downward slope. When the vehicle goes uphill, the vehicle body load on the air suspensions on the rear-wheel side increases, while that on the air suspensions on the front-wheel side decreases. When the vehicle goes downhill, on the other hand, the vehicle body load on the front-wheel air suspensions increases, while that on the rear-wheel air suspensions decreases.
The increase and decrease of the vehicle body load on the air suspensions on the front- and rear-wheel sides cause the proper steering characteristic of the motor vehicle to change. More specifically, when the vehicle goes uphill, its steering characteristic shifts to the under-steering side. When the vehicle goes downhill, on the other hand, its steering characteristic shifts to the over-steering side.
If the steering characteristic of the motor vehicle is deviated from the proper one due to the gradient of the road, therefore, it cannot be effectively varied even though the roll controls for the front and rear portions of the vehicle body are independently executed with the delay time between them, as mentioned before.
According to the conventional suspension system, moreover, the roll controls for the front and rear portions of the vehicle body are executed without giving consideration to the bent state of the road, that is, the curvature of corners or frequency of cornering, associated with the turn of the motor vehicle, in addition to the gradient of the road. Thus, the steering characteristic of the motor vehicle cannot be optimally varied depending on the behavior of the vehicle.